A lithographic apparatus is a machine that applies a pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) of a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Imprinting may be carried out in a step-like way, wherein a stationary substrate is imprinted with a pattern from a stationary patterning device. An alternative way comprises scanning the substrate with a patterned beam. One of the known ways to do this is by way of pulsed illumination of the patterning device, and thus of the substrate. The image on the substrate may thus be built up of many pulsed illuminations of the patterning device, in such a way that the pulsed images overlap locally, and a sharp image is formed. This scanning type of illumination will be discussed more extensively below.
A problem with scanning type of illumination is that there typically are variations in the intensity of the radiation beam. These differences entail that different parts of the target portion of the substrate may receive a different total illumination dose, which dose differences may become visible e.g. as feature variations that extend in a direction perpendicular to the scanning direction. Especially for pulsed illumination, these differences may become significant, as for example an incorrect scan speed with respect to the scanning beam may result in different portions of the substrate actually being illuminated by a different number of pulses. Furthermore, pulse-to-pulse variations may also give rise to inhomogeneous illumination. While the prior art has attempted to address this issue, such attempts have raised other undesirable consequences.